The present invention relates to a converter secondary circuit having an auxiliary output.
A converter usually receives electrical power from a power supply having an input voltage and delivers electrical power to a load at an output voltage. The voltage at the main output is regulated by the primary of the converter to guarantee that a constant voltage is applied to the load even if the input voltage and/or the output current vary. It is often desirable to add one or more auxiliary outputs, which must be regulated independently by a secondary circuit.
The patent document EP 0 954 088 describes a secondary circuit for a converter having a main output and an auxiliary output. That converter comprises:
a first synchronous rectifier which is self-controlled, that generates a first rectified voltage for the main output and receives at its input a voltage supplied by a first transformer, and
a second synchronous rectifier that generates a second rectified voltage for said auxiliary output and that receives at its input a voltage supplied by a second transformer whose primary is the choke for filtering the first rectified voltage.
The second synchronous rectifier is controlled by an auxiliary regulation control circuit having two terminals connected to respective terminals of the secondary of the first transformer to synchronize the second rectifier.
FIG. 1 shows a converter secondary circuit with a main output, described in patent document EP 0 618 666.
An AC signal from the primary is present at the terminals 1A and 1B of the transformer 1. This signal is rectified by a rectifier and filtered by a filter to obtain the required DC voltage at the terminals 2A and 2B of the main output 2.
The self-controlled synchronous rectifier comprises a freewheel MOSFET 3 and a direct MOSFET 4 and the filter is an LC circuit 5, 6. Synchronous rectification using MOSFETs considerably reduces losses. The gate of the freewheel transistor 3 is connected to the terminal 1B of the transformer 1 and the gate of the direct transistor 4 is connected to the terminal 1A of the transformer 1. Because of these connections, switching of the transistors 3 and 4 is automatically synchronized with the primary circuit.
The above kind of converter secondary circuit can be provided with an auxiliary output 7.
FIG. 1 shows a prior art auxiliary output circuit.
The auxiliary output 7 also has a controlled synchronous rectifier consisting of a direct MOSFET 9 and a freewheel MOSFET 10 and a filter consisting of an LC circuit 11, 12 and is regulated by an auxiliary regulation control circuit 8.
The auxiliary regulation control circuit 8 has a control input 8A connected to the terminal 7A of the auxiliary output 7, a synchronization input 8B connected to the terminal 1A of the transformer 1, and two outputs 8C and 8D connected to the gates of the transistors 9 and 10, respectively.
The auxiliary regulation control circuit creates a time-delay for controlling the energy transmitted on each switching cycle. This time-delay is a function of the control voltage VCmd.
To avoid the need for a bidirectional switch, it is the unidirectional voltage VRect after rectification in the secondary circuit that feeds the auxiliary output 7 via the MOSFETs 9 and 10 and the LC filter 11, 12.
FIG. 3 shows the theoretical waveforms of the signals in the above kind of circuit.
The voltage at the drain of the direct transistor 9 is the voltage VRect rectified by the secondary circuit. An integrator converts the output voltage into a DC voltage VCmd and, by comparison with a voltage ramp VRmp, a binary signal VCtrl is obtained that controls the turning on and off of the transistors 9 and 10, as shown by the graphs QD and QRL. The resulting voltage VDly is therefore shifted by a particular time-delay before it is filtered to obtain the voltage at the auxiliary output 7.
The means used to achieve this are known in the art and are explained in more detail in the description given hereinafter.
The voltage ramp VRmp is synchronized to the voltage at the terminal 1A of the transformer 1 and therefore to voltage VRect.
This represents the ideal operation. In reality, on each switching of the direct transistor 9 of the auxiliary output synchronous rectifier, an unwanted voltage drop occurs in the rectified voltage VRect, as shown in FIG. 4. This unwanted voltage drop leads to parasitic synchronization of the voltage ramp VRmp and to poor operation of the circuit, the output voltage falling and the converter ceasing to function.
One solution would be to add a filter to cancel this parasitic effect. However, the overall efficiency of the converter would then decrease because of the time-delay introduced by the filter. Moreover, if the auxiliary output current is too high, a filter of that kind is no longer effective.
The invention solves this problem and, to do so, proposes a converter secondary circuit having a main output and an auxiliary output, the secondary circuit comprising a synchronous rectifier which is self-controlled and generates a-rectified voltage for the main output and said auxiliary output including a second synchronous rectifier which receives at its input said rectified voltage and is controlled by an auxiliary regulation control circuit having a synchronization input, which circuit is characterized in that said synchronization input receives a synchronization signal which is at zero when said rectified voltage is not at zero.
If the first synchronous rectifier consists of a main MOSFET and a freewheel MOSFET, the gate of the freewheel transistor is connected to a terminal of a transformer at a synchronization point, said synchronization input of said auxiliary regulation control circuit is connected to the secondary circuit, and said synchronization input is connected to said synchronization point.
Said synchronization input is advantageously connected in the auxiliary regulation control circuit to a rising edge detector, a falling edge detector or a level detector.
Said second synchronous rectifier preferably consists of a direct MOSFET and a freewheel MOSFET.